Watch your dog the next time you go for a walk or send him into the yard. Within moments of being outside, his nose immediately goes to work tracking squirrels, rabbits and the meter man. Airflow, moisture, temperature and density all influence how easily he tracks a smell.
The dog's nose is an amazing feat of evolution. Capable of distinguishing more than 1,000 individual chemical odorants, the dog's nose surpasses the human nose at least 10,000 times over. In addition to a proportionately large nasal cavity, the dog also possesses a special organ called the vomeronasal organ. The VMO helps the dog distinguish between organic smells and inorganic smells. For organic smells, it provides biological information on species, gender, health status, mood and more, giving the dog a complete picture of the animal they are smelling.
Fill a glass with water, then place two to three drops of food coloring into the glass. Watch how it changes. This is how scent travels in air. It moves from high to low elevation, gets caught in swirls of movement and dissipates as it moves further away from the origination point.
Temperature and moisture affect the strength of scents as well as how quickly they dissipate. When the temperature drops, air and scent molecules become more dense, resulting in a higher concentration of smell per square inch. This is because scent molecules move closer together when cold. Similarly, water vapor traps scent molecules, enhancing the odor they release and retarding their diffusion into the air. The combination of cool, moist air results in scent that is heavy and closer to the ground. This makes it much easier for a dog to track the smell.
While it is true that cool, moist air makes it easier for dogs to smell and track odor, it is also true that air can be too cold for scent. This is because as temperatures continue to drop, the moisture in the air freezes to a solid. Eventually, the scent becomes trapped in dry solids and cannot be detected easily by the dog. It is for this reason that dogs may have trouble tracking in subzero temperatures but do well on cool, rainy days.
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